Water-swellable polymers are used as constituents in personal care devices which absorb bodily fluids, such as sanitary napkins, incontinent devices and disposable baby diapers. Examples of such polymers are disclosed in U.S. Pat. Nos. 3,926,891; 4,190,562; and 4,293,609.
Various processes are known for producing absorbent polymers. For example, U.S. Pat. No. 4,833,222 teaches a process for preparing an absorbent polymer from a neutralized monomer with a surface-active agent. U.S. Pat. No. 4,808,637 teaches the uniform reaction of acrylic acid, an alkali metal salt of carbonic acid, aluminum acetate, sodium sulfate and water, preferably using microwave radiation as a heat source to initiate polymerization.
Another process comprises the steps of preparing a reaction mixture consisting essentially of particular amounts of polymerizable acid group-containing monomers, crosslinking agent and optionally free radical initiator in an aqueous medium and thereafter polymerizing and neutralizing at least a portion of the acid functional groups.
Absorbent polymers used in personal care devices are characterized by certain performance properties, including centrifuge capacity, absorbence under load, shear modulus, and percent extractables.
U.S. Pat. No. 4,666,983 discloses an absorbent article obtained by mixing 100 parts by weight of an absorbent resin powder having a carboxyl group with 0.0001 to 10 parts by weight of a crosslinking agent to crosslink the molecular chains existing at least in the vicinity of the surface of the absorbent resin powder. The patent discloses that at least 70 percent by weight of the particles will preferably pass through a 60-mesh sieve. Comparative Example 2 of the patent provides that when the crosslinking agent was added to a gel-like hydrous polymer having a larger particle size than the powder particles, the effect was contrary to that expected, or was small.
U.S. Pat. No. 4,734,478 discloses a water-absorbing resin powder having the molecular chains crosslinked near the particle surfaces, the resin powder being obtained by mixing 100 parts by weight of a powder of a carboxyl-containing water-absorbing resin with 0.001 to 10 parts by weight of a polyhydric alcohol and heating the mixture at a temperature of at least 100.degree. C. to react the powder with the polyhydric alcohol, the improvement comprising conducting the mixing of the powder and the polyhydric alcohol in the presence of 0.01 to 8 parts by weight of a hydrophilic organic solvent and 0 to 8 parts by weight of water per 100 parts by weight of the powder. The patent provides that not more than 50 percent by weight of the particles of the resin powder will pass through a 200-mesh sieve.
The process incorporated into the product claims of U.S. Pat. No. 4,734,478 suffers the following disadvantages. First, traces of water are quickly absorbed by water absorbent polymers, making it difficult to introduce additives to the polymer particles, which additives are to be applied as an aqueous solution. Second, the addition of an aqueous solution to the polymer powder results in particle agglomeration; when the agglomerated particles are subjected to heat, they may form hard clusters which make additional processing such as breaking and screening necessary. Third, the patent discloses that if the polyalcohols are added without a solvent, good distribution is not possible. The polymer coating formulation therefore preferably contains a hydrophilic organic solvent, e.g., methanol. The use of an organic solvent is not preferred, due to the associated special handling and the solvent recycle/disposal requirements.
U.S. patent application Ser. No. 07/866,628 discloses a process for preparing a surface crosslinked water absorbent polymer, wherein a hydrogel is contacted with a composition containing a polyhydroxy compound and a surfactant to coat the hydrogel. The coated hydrogel is then dried to form particles, is optionally ground to form sized particles, and is heated to crosslink the surface of the particles. In preferred embodiments, the hydrogels will comprise from about 15 to about 100 weight percent polymer, with the remainder comprising water. In preferred embodiment, the hydrogel is preferably in granular form, with particle sizes of about 2 cm or less being more preferred. The composition which contains the polyhydroxy compound and the surfactant will preferably contain from about 0 to about 99 percent water and from about 0 to about 50 weight percent organic solvent. The resultant compositions are disadvantageous to the extent that they require the use of expensive surfactants, which add cost to the finished product.
Japanese Kokai Publication 84632/1981 discloses an absorption agent consisting of a crosslinked poly(alkali metal acrylate) material obtained from 0.01 to 10 parts by weight of a water-soluble and/or water dispersible surfactant and 0.005 to 20 parts by weight of a water-soluble polyvalent alcohol per 100 parts by weight of an acrylic acid/acrylate based polymer composed of from 10-40 mole percent acrylic acid and 60-100 mole percent acrylate salt, wherein the polymer is formed by gel polymerization as a gel which is subsequently heated and dried.
European Patent Application 248,963 discloses a process of post treatment wherein the surface of water-absorbent resin particles are treated with polyquarternary amines to significantly increase the absorption rate and to increase the absorption under load (AUL) by 10 percent. In the disclosed process, the polyquarternary amines are applied as solutions in methanol. A separate blending operation is required to evenly distribute the polyquaternary amine evenly throughout the resin.
European Patent Application 248,437 discloses a process for post surface crosslinking in which an aqueous solution of a water-soluble peroxide radical initiator is sprayed onto the surface of the water absorbent resin particles and the coated particles are heated. The reference alleges the achievement of additional surface crosslinking, leading to a product of improved water absorbency and water absorption rate. The reference discloses that the uniformity of penetration of the aqueous solution into the surface of the absorbent polymer may be improved by using a water soluble organic solvent such as methanol. The disclosed process suffers the disadvantage of using high levels of peroxide free radical initiators, which discolors partly neutralized polyacrylic acid, making the product less appealing for personal care applications. The disclosed process suffers the further disadvantage of increasing the amount of extractable polymer, e.g., low molecular weight water-soluble polymer, if the polymer is subjected to heat.
German Patent DE 3,713,601 discloses a process in which surface crosslinking is obtained by the addition of a crosslinker of glycidyl or polyglycidyl compounds. These crosslinking agents are not preferred in applications wherein the polymer is expected to contact human skin.
In the prior art processes, it has been noted that the contact time of the polymer and the surface crosslinking agent should be sufficient to coat the particles with the surface crosslinking agent, but not so long as to allow diffusion of the surface crosslinking agent into the pores or internal structure of the water-absorbent resin particles. It would be advantageous to provide a process which is flexible with respect to the amount of time for which the polymer may be in contact with the surface crosslinking agent prior to the onset of the surface crosslinking reaction.
Industry would find great advantage in a process for preparing an aqueous fluid absorbent having improved absorbency under load and capacity, which process does not require the use of expensive surfactants and which process does not require the implementation of additional process equipment. Such a process would provide a highly desirable product in a cost-effective manner.